fix: updating preprocess_schemas to create operation schemas for extensions

[ryanmco]

Description

The function get_required_ops(schema) only scans the top-level properties of a schema file to determine whether to generate *_create_request.json, *_update_request.json. Therefore, there is no fulfillment_create_request.py created with a CreateFulfillmentRequest type. This fixes that so FulfillmentCreateRequest can be used instead of FulfillmentObject when composing with CheckoutCreateRequest.

Category (Required)

Please select one or more categories that apply to this change.

• Core Protocol: Changes to the base communication layer, global context, or breaking refactors. (Requires Technical Council approval)
• Governance/Contributing: Updates to GOVERNANCE.md, CONTRIBUTING.md, or CODEOWNERS. (Requires Governance Council approval)
• Capability: New schemas (Discovery, Cart, etc.) or extensions. (Requires Maintainer approval)
• Documentation: Updates to README, or documentations regarding schema or capabilities. (Requires Maintainer approval)
• Infrastructure: CI/CD, Linters, or build scripts. (Requires DevOps Maintainer approval)
• Maintenance: Version bumps, lockfile updates, or minor bug fixes. (Requires DevOps Maintainer approval)
• [x ] SDK: Language-specific SDK updates and releases. (Requires DevOps Maintainer approval)
• Samples / Conformance: Maintaining samples and the conformance suite. (Requires Maintainer approval)
• UCP Schema: Changes to the ucp-schema tool (resolver, linter, validator). (Requires Maintainer approval)
• Community Health (.github): Updates to templates, workflows, or org-level configs. (Requires DevOps Maintainer approval)

Related Issues

Checklist

• [ x] I have followed the Contributing Guide (including Conventional Commits title requirements and ! for breaking changes).
• I have updated the documentation (if applicable).
• [ x] My changes pass all local linting and formatting checks.
• [ x] I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• (For Core/Capability) I have included/updated the relevant JSON schemas.
• I have regenerated Python Pydantic models by running generate_models.sh under python_sdk.

Screenshots / Logs (if applicable)